This was a very difficult question for me; you have Bach who is widely considered the "old-testament" of keyboard music, you have Mozart who defined clcassical piano with his concertos, and you have Liszt and Chopin who are the epitome of Romantic piano.
Ultimately I have to give the the nod to Beethoven. He transcended genres, and I truly think he is the greatest musical mind to ever grace this earth. The Diabelli Variations combined with the 32 Piano Sonatas stand on their own special level, and I consider them unrivaled.
That being said, I have to give honorable mentioned to Liszt. I think too often he is denied the credit he deserves for really making the solo piano important, and his catalog is munmental, and loaded with masterworks. I have the Leslie Howard's complete Liszt recordings, which span about 100 discs, and they simply leave me in awe.